Conventional snow tires provide grip on icy road surfaces via stud pins mounted in a tread portion of the tire.
Typical stud pins are embedded in a stud pin installation hole provided in the tread portion. When stud pins are embedded in a stud pin installation hole, the stud pin installation hole has an expanded diameter. This firmly embeds the stud pin in the stud pin installation hole. As a result, stud pins are prevented from falling out from the stud pin installation hole upon receiving forces upon breaking or accelerating or lateral forces from the road surface when the tire rolls.
The stud pin is provided with a buried base portion, and a tip portion that projects beyond an end surface of the buried base portion. The buried base portion is embedded in the stud pin installation hole formed in the tread surface of the tire so that the tip portion projects from the tread surface.
The edges of the tip portion of the stud pin come into contact with the icy road surface, providing an edge effect that provides a high gripping force. Accordingly, effort has been made towards increasing the edge effect by providing the tip portion with more edges that come into contact with the icy road surface.
A stud pin is known that includes a tip portion with a concave polygonal end surface and a recessed portion in a side surface for increasing the edges of the tip portion (for example, see International Patent Application Publication No. WO 2014/122570). Additionally, effort has been made towards increasing the edges and thus increasing the edge effect by making the tip portion larger.
However, when a tire mounted with a stud pin that has a concave polygonal end surface runs on icy road surfaces, ice shaved by the tip portion when the tire is driven builds up in the recessed portion of the tip portion. A build-up of fine ice particles in the recessed portion may reduce the ability of the tip portion to break up ice when the tire is driven, and thus reduce driving performance.
However, increasing the edges of the tip portion results in an increase in the amount of wear of the road surface when running on a non-icy road surface.